The present invention relates to a method of and an apparatus for controlling heat transfer in a fluidized bed reactor, according to the preambles of appended independent claims.
The present invention particularly relates to a method and an apparatus for recovering heat from solid particles in a fluidized bed reactor comprising a processing chamber, having a fluidized bed of solid particles therein, and a heat transfer chamber, being in solid particle communication with the processing chamber and having heat transfer surfaces disposed therein. The heat transfer chamber may be connected in various ways to the processing chamber so that there is solid particle exchange between the chambers. The heat transfer chamber may in some special case even be formed within the processing chamber itself.
The present invention relates to a method and apparatus applicable in atmospheric, as well as, pressurized fluidized bed reactor systems.
Fluidized bed reactors, such as circulating fluidized bed reactors, are used in a variety of different combustion, heat transfer, chemical or metallurgical processes. Typically heat, originating from combustion or other exothermic processes, is recovered from the solid particles of the fluidized bed by using heat transfer surfaces. Heat transfer surfaces conduct the recovered heat to a medium, such as water or steam, which transports the heat from the reactor.
Said heat transfer surfaces are usually located in the. processing chamber or within a convection section arranged in the gas pass after the processing chamber or, in circulating fluidized bed reactors, within a particle separator. Additional heat transfer surfaces may be arranged in separate heat transfer chambers (HTC), which may be a part of the processing chamber, a separate chamber adjacent to the processing chamber or, in circulating fluidized bed reactors, part of the solid particles recycling system.
In a heat transfer chamber (HTC), heat is typically recovered by continuously introducing hot solid particles from e.g. the processing chamber into the HTC, recovering heat from said solid particles in the HTC, and continuously discharging said solid particles from the HTC into the processing chamber. Said heat recovery takes place by using heat transfer surfaces disposed in the HTC.
The HTC thereby comprises inlet means for introducing a continuous flow of hot solid particles from the processing chamber into the HTC, heat transfer surfaces and means for transporting the heat recovered from the hot solid particles out from the HTC, and outlet means for continuously recycling solid particles discharged from the HTC into the processing chamber.
Accurate and fast controllability of the heat transfer is an important consideration in many applications of fluidized bed reactors, such as where maintaining constant steam temperature may require rapid and accurate adjustments of heat transfer. The reason for the need of controlling action may be a changing demand of the produced steam or abnormality in the fuel quality or fuel feed or some other abnormality in the system. Also there may be a need to adjust the system to proper operating state. In steam boilers, additional requirements to adjust the heat transfer arise from the fact that heat is usually recovered in many stages, i.e. in evaporators, superheaters, economizers and reheaters, which may need independent control,
From the point of view of the processes in a fluidized bed reactor, the aim of the heat transfer control is to maintain optimum performance, especially taking into account the harmful emissions or combustion efficiency. Usually, this implies that the temperature of the reactor should stay constant, even in conditions of varying heat recovery and fuel input rates.
In circulating fluidized bed reactors, the rate of heat recovery in the upper parts of the furnace can be varied by changing the bed density. This can be realized by collecting part of the bed material to a storage, as shown in U.S. Pat. No. 4,823,739 or, more simply and quickly, by changing the fluidizing gas velocity. However, the fluidizing gas is an important factor in reactions taking place in the processing chamber of the circulating fluidized bed reactor. To maintain an economically and ecologically favorable operation, changes in the fluidizing gas require other simultaneous changes, such as changes in the fuel feed rate. Thus, this method of heat transfer control effects all heat transfer surfaces of the system and can be favorably put into effect only in the time scale of the thermal time constant of the whole system.
Due to the large heat capacitances involved, the thermal time constant of a fluidized bed reactor, i.e. the time when, after a step-wise stimulus, approximately two thirds of its temperature change has taken place, can be very long, e.g. 25 minutes. Thus, the heat transfer from a fluidized bed based on heat transfer surfaces having an invariable thermal contact to the bed is not fast enough for many applications of fluidized bed reactors.
To render possible a fast control of the heat transfer from fluidized bed reactors, with time constant of e.g. some tens of seconds, different constructions utilizing separate HTCs have been developed. Because also in a HTC the temperature of the solid particles can vary only slowly, different techniques have been developed, which do not depend on varying the temperature of said solid particles to control the heat transfer.
The simplest means for such control is to vary the amount of hot material in contact with the heat transfer surfaces in the HTC so that only a variable part of the heat transfer surfaces are covered by the solid particles. This kind of construction was disclosed e.g. in U.S. Pat. No. 4,813,479. However, to control the level of solid particles at least one additional flow duct and a controlling valve is needed, which increases the complexity and costs of the system.
Another approach, which has been used in circulating fluid bed reactors, is to divide the flow of hot solid particles after the particle separator to two channels, of which only one has heat transfer surfaces. Thus, when varying the division ratio of solid particles flowing through said two channels, the rate of heat transfer is varied. In order to function properly, this technique also requires a rather complicated construction, such as that disclosed in U.S. Pat. No. 5,140,950, in which many compartments and channels are used.
HTCs are normally bubbling fluidized beds with low gas flow velocities, e.g. from 0.1 to 0.5 m/s. The transport of solid particles through a HTC or through its different channels can be controlled by mechanical valves or by varying the fluidizing gas velocity, and thereby the bed height, in different portions of the HTC.
It is known that the heat transfer coefficient of a heat transfer surface in a fluidized bed can to some extent be varied by changing the velocity of fluidizing gas flow. (The heat transfer coefficient refers to the amount of thermal energy transferred across one square meter of the heat transfer surface per one degree temperature difference between the bed and the medium transporting the heat away.) This is due to the fact that with higher velocities of fluidizing gas flow, the solid particle movements are more intense and give a more uniform temperature distribution in the fluid bed, and, thus, the heat transfer on the heat transfer surfaces is enhanced.
Because, in typical HTC constructions, the gas flow velocities are related to the particle flow, they cannot be independently varied. U.S. Pat. No. 5,425,412 discloses an arrangement in the return duct of a circulating fluidized bed reactor, where the HTC contains a separate heat transfer section where the gas flow velocity can be varied independently from the particle flow. Moreover, U.S. Pat. No. 5,406,914 discloses another arrangement with a separate heat transfer section which has also an additional passage for particles directly from the processing chamber to the HTC. With a similar principle can also be constructed a separate HTC, with a heat transfer gas flow which is independent of the particle transfer gas flow.
However, at least when high turn down ratio is needed, the methods disclosed in U.S. Pat. Nos. 5,425,412 and 5,406,914 do not provide ideal control of heat transfer, because in bubbling fluidized beds the heat transfer coefficient typically changes from a low value to a much higher value rather abruptly within a narrow fluidizing gas flow velocity range. Thus, by using the flow velocity as a control parameter, it is not possible to achieve a smooth and continuously controllable operation on a large control range.
It has been suggested in GB patent publication 929,156 a method of heat exchange between a fluid circulating through a heat exchange surface and a granular powdered material by fluidizing the granular powdered material in a pulsating manner. Thereby, during the time of each pulse, the injection of fluidization gas is effected at a velocity which is similar or higher than that of the velocity of fluidization gas during continuous fluidization, whereas no injection of fluidizing gas takes place between the pulses. The periodic pulsations comprise relative short injection periods alternating with longer stationary periods. Thereby two problems mentioned in the GB patent are dealt with. The quantity of fluidizing gas volume is decreased and the granular material is maintained f or longer periods in a state of maximum density.
The method suggested in GB 929,156 does not, however, appear to be applicable in fluidized bed reactors, in which as stable as possible process conditions are desired. All kinds of variations having a negative influence on the process have to be avoided. The great variation in fluidization velocity, i.e. between zero fluidization and a velocity similar to or higher than conventional fluidization velocity, may have such a negative impact on other process variables.
A further drawback of the suggested heat transfer control based on pulsed gas flow velocity, including zero gas flow velocities, is the poor mixing of the solid particles in the HTC. Particularly if after burning takes place, there is a risk of overheating and agglomeration of bed material at some locations of the HTC. It has also been noticed that nozzles in the bottom of a fluidized bed reactor have a tendency to leak unless a minimum gas velocity is maintained in the reactor.
It is an object of the present invention to provide a method and apparatus for controlling heat transfer in fluidized bed reactors in which the above mentioned drawbacks have been minimized.
It is especially an object of the present invention to provide a method and apparatus in which heat transfer in a HTC in a fluidized bed reactor can be controlled rapidly and accurately over a large control range, even at low heat transfer rates without a risk of overheating.
The present invention provides an improved method of and apparatus for controlling heat transfer in a fluidized bed reactor, which includes a heat transfer chamber (HTC) with a bed of solid particles therein, means for continuously, i.e. substantially uninterruptedly, introducing fluidizing gas into the heat transfer chamber for fluidizing said bed of solid particles therein and heat transfer surfaces in contact with said bed of solid particles in said heat transfer chamber, the heat transfer surfaces recovering heat from solid particles in the heat transfer chamber. The fluidized bed reactor according to the present invention further comprises means for varying the continuous fluidization of the bed of solid particles in the heat transfer chamber according to a periodical function. The flow of fluidizing gas, which is continuously introduced into the heat transfer chamber, is periodically varied between two or more different flow velocities, for controlling the instantaneous heat transfer from solid particles to said heat transfer surfaces in said heat transfer chamber.
The effective or average overall heat transfer in the heat transfer chamber is thereby controlled by varying a parameter of the periodically varying flow of fluidizing gas being introduced into the heat transfer chamber.
The present invention is applicable in different types of fluidized bed reactors, such as in bubbling fluidized bed reactors or in circulating fluidized bed (CFB) reactors. Thereby the fluidized bed reactor comprises a processing chamber, such as a combustion chamber, in solid particle flow communication with the heat transfer chamber. Heat generated in said processing chamber is thereby recovered with the heat transfer surfaces in the heat transfer chamber. The heat transfer chamber, in which heat transfer is controlled according to the present invention, may even be an integral part of a bubbling fluidized bed reactor, i.e. at least a portion of the bubbling bed itself may form a heat transfer xe2x80x9cchamberxe2x80x9d zone.
The invention is also applicable in fluidized bed ash coolers, cooling ash and/or other bed material discharged from the combustion chamber of a fluidized bed reactor. The heat transfer chamber may be connected to a CFB as an external heat exchanger in the solid material recirculation loop or as an internal heat exchanger connected to the internal bed material circulation.
Thereby, according to a preferred embodiment of the present invention, there is provided an improved method of recovering heat from solid particles in a fluidized bed reactor, utilizing a HTC, comprising the steps of:
continuously introducing hot solid particles from the processing chamber into the HTC, and continuously discharging said solid particles from the HTC into the processing chamber,
recovering heat from said solid particles in the HTC by heat transfer surfaces,
varying the flow velocity of fluidizing gas introduced into the HTC according to a periodic function.
Additionally, the method may have the steps of:
observing a possible need to change the heat transfer rate, e.g. by monitoring the temperature of the medium which convects the recovered heat from the HTC, and
varying a parameter of the flow velocity of the fluidizing gas, in the heat transfer chamber, to change the effective heat transfer rate in accordance with observed need,
There is also according to a preferred embodiment of the present invention, provided an improved apparatus for recovering heat from solid particles in a fluidized bed reactor, utilizing a HTC, said apparatus comprising:
means for continuously introducing solid particles from the processing chamber into the HTC and means for continuously discharging said solid particles from the HTC into the processing chamber
heat transfer surfaces for recovering heat from said solid particles and means to transport the recovered heat from the HTC
means for providing periodically varying flow of fluidizing gas.
Additionally, the apparatus may have:
means for observing a possible need to vary the heat transfer rate and
means for varying a parameter of said periodically varying flow of fluidizing gas.
According to one aspect of the present invention the flow velocity of a continuous flow of gas is alternated between a first and a second flow velocity, said first flow velocity being higher than the second velocity, said first flow velocity thereby providing a higher instantaneous heat transfer from the solid particles to the heat transfer surfaces than the second flow velocity. The flow velocity may, if needed or otherwise desired, be alternated between more than two different flow velocities.
When the flow velocity is alternated between predefined constant values, the lowest velocity should still exceed zero. The lowermost flow velocity should mainly be set to a level which prevents the fluidization nozzles from leaking or being blocked by particles falling into or on the nozzle openings.
In systems where the fluidization gas of the HTC provides a considerable part of the combustion air into the system a minimum flow velocity of gas/air has to be maintained in order not to cause instability to the system. Also a certain minimum flow velocity of air may be needed in order to prevent reducing conditions and increased corrosion attack on heat transfer surfaces from arising.
Due to above mentioned reasons, the minimum value of the periodically varying fluidization gas flow velocity in the HTC should preferably be maintained at a level corresponding to at least 10%, preferably at least 20%, of the maximum value of the periodically varying fluidization gas flow velocity in the HTC.
The flow velocity of the gas being introduced into the heat transfer chamber may also be periodically varied according to e.g. a step-wise function, a saw-tooth function, a sin-function or the like. The form of the function, as such, normally is not important. The form of the function generally depends on the construction of the means providing the periodically varying flow of fluidizing gas and/or means for varying a parameter of the periodical flow of fluidizing gas. In order to be able to vary the average flow of fluidizing gas and the effective heat transfer rate, it should be possible to vary at least one of the parameters of the function.
As mentioned earlier, the heat transfer coefficient in a bubbling fluidized bed typically changes from a low value to a much higher value rather abruptly within a narrow fluidizing gas flow velocity range. The heat transfer coefficient reaches a maximum at a certain flow velocity and decreases again at higher flow velocities. In the following, the fluid velocity range, where the heat transfer coefficient for instantaneous heat transfer changes from 60% of its maximum to 80% of its maximum, is called the xe2x80x9ctransition velocity rangexe2x80x9d and the fluid flow velocities lower than the xe2x80x9ctransition velocity rangexe2x80x9d are called xe2x80x9clowxe2x80x9d velocities and fluid flow velocities higher than the xe2x80x9ctransition velocity rangexe2x80x9d are called xe2x80x9chighxe2x80x9d velocities.
In a preferred embodiment said periodically varying fluidizing gas flow velocity depends on time as a time dependent step-function, i.e. a function, the value of which alternates between two constants, one of which represents a xe2x80x9clowxe2x80x9d velocity and the other a xe2x80x9chighxe2x80x9d velocity. Thus, the parameters of the periodical flow are the durations and velocities of the xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d parts of a flow period. During the sub-period when the velocity is xe2x80x9chighxe2x80x9d, there is high instantaneous heat transfer rate in the heat transfer surfaces. When the velocity is xe2x80x9clowxe2x80x9d the instantaneous heat transfer is low. When the periodical function is such, that the bed is always or most of the time in the xe2x80x9chighxe2x80x9d state, the effective or average overall heat transfer rate is high. When the proportion of the xe2x80x9chighxe2x80x9d velocity is small, the effective heat transfer rate is low.
The periodically varying gas flow velocity function does not necessarily have to be a step-function alternating between two constants, but can, if desired, be another suitable time dependent function, however, preferably varying within a range limited by a xe2x80x9clowxe2x80x9d and a xe2x80x9chighxe2x80x9d velocity, the xe2x80x9clowxe2x80x9d and xe2x80x9chighxe2x80x9d velocities preferably being predetermined.
FIG. 1a schematically illustrates a periodical, step-wise alternating flow function. t1 is the duration of the xe2x80x9chighxe2x80x9d flow velocity and t2 that of the xe2x80x9clowxe2x80x9d flow velocity. At points denoted by i) and ii) in FIG. 1a, as also in other examples 1b, 1c and 1d, the effective heat transfer rate increase from a low value to an intermediate value and from an intermediate value to a high value.
At atmospheric pressure the transition flow velocity range, separating regions of high and low heat transfer, is typically near 0.2 m/s for fine bed material and between 0.4 and 0.5 m/s for coarse bed material. Thus at ambient pressure for fine bed material, such as bed material circulating in a CFB reactor, xe2x80x9chighxe2x80x9d flow velocities, or the upper limit for the flow velocity, may be e.g. xe2x89xa70.2 m/s, typically xe2x89xa70.25 m/s. The difference between the xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d flow velocity being  greater than 0.1 m/s, preferably  greater than 0.15 m/s. Similarly, at ambient pressure for coarse bed material, such as ash material discharged from the lower part of a CFB reactor, the xe2x80x9chighxe2x80x9d flow velocities may be e.g. xe2x89xa70.4 m/s, typically xe2x89xa70.5 m/s. The difference between xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d flow velocities being  greater than 0.2 m/s, preferably  greater than 0.25 m/s respectively. The difference between the xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d flow velocity should, however, not exceed the value of the xe2x80x9chighxe2x80x9d flow velocity, as a minimum xe2x80x9clowxe2x80x9d flow velocity is to be maintained at all times, in order to prevent problems, such as agglomeration, leakage of nozzles, blockage of nozzles and instability in the system, arising with zero or too low flow velocities.
It has, however, been noted that the xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d flow velocity values strongly depend on pressure. Thus at elevated pressures, e.g. at 10 bar, the above mentioned flow velocity values may have to be divided by two or more.
The periodically varying instantaneous heat transfer coefficient of heat transfer surfaces in a fluidized bed depends besides on pressure, on the size, roundness and density of the bed particles. Thus according to one aspect of the present invention the flow velocity range, which separates xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d flow velocity values, may be hard to define as a velocity range. Alternatively, the range may be defined as the range, where the instantaneous heat transfer coefficient changes from 60% to 80% of its maximum value. The maximum value of instantaneous heat transfer being the maximum value practically obtainable by the specific heat transfer surfaces in the specific heat transfer chamber.
In FIG. 1a, so as also in other examples 1b, 1c and 1d, the broken horizontal line represents approximately the flow velocity range, which separates the regions of high and low heat transfer coefficients.
In a preferred embodiment the duration of the xe2x80x9chighxe2x80x9d flow velocity sub-period is constant, e.g. 2.0 s, and the duration of the xe2x80x9clowxe2x80x9d flow velocity sub-period may for heat transfer control purposes be varied between certain values, say 0 s and 10 s, to cover the desired heat transfer range. If desired the duration of xe2x80x9clowxe2x80x9d flow velocity sub-periods may be constant and the duration of xe2x80x9chighxe2x80x9d flow velocity sub-periods may be varied, or duration of both sub-periods may be varied. Sufficient mixing of the bubbling bed requires xe2x80x9chighxe2x80x9d velocity sub-periods within certain not too long intervals, these intervals should typically not exceed 30 s. Also, to avoid detrimental periodical variations of the temperature of the heat transfer medium or of the reactor, the sub-periods should in most cases be shorter than the corresponding thermal time constants of the system.
To filter out possible periodic thermal disturbances, e.g. cyclically varying steam temperature, the HTC may have two or more zones with separately controlled fluidizing gas inlets or two or more sets of separately controlled fluidizing gas inlets within the same area. The xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d flow velocity sub-periods may be arranged to occur in the separately controlled fluidizing gas inlets at different times in different zones or different sets of inlets. When fluidizing gases to the different inlets have their xe2x80x9chighxe2x80x9d flow velocities at different times, the risk for cyclical temperature variations is minimized. If the HTC has N zones, the periodic flow velocities are preferably operated with 360 degrees / N phase differences.
The periodic flow function may be of many other forms than the function described above. Because the effective heat transfer rate is in practice a complicated function of many parameters, any one of the parameters or any combination of them can be used as control variables. FIG. 1b shows, as another example, a step function, where the ratio of the durations of the xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d sub-periods is used as a control parameter.
The periodical flow function does not have to be a step-function, but it can be, e.g., a sine-function or a sawtooth-function with a variable offset or a sawtooth-function with a variable amplitude. FIGS. 1c and 1d show, as further examples, a sin-function and a sawtooth-function, which could be used as periodic flow functions.
In test runs performed, the effective heat transfer coefficient of the heat transfer surfaces in a HTC varied typically between 100 W/m2K and 400 W/m2K. In these runs, the periodical fluidizing gas flow velocity was of the type shown in FIG. 1a. The duration of the xe2x80x9chighxe2x80x9d flow sub-period was kept constant, typically 1 s, and that of the xe2x80x9clowxe2x80x9d sub-period was varied. With 10 s xe2x80x9clowxe2x80x9d sub-period duration, the heat transfer constant was 100 W/m2K and with 0 s it was 400 W/m2K. The heat transfer coefficient varied with intermediate durations substantially linearly between these extreme values. A useful control range from 100% to 25% was thus obtained.
An additional feature of this invention is that based on a need, which is observed e.g. by monitoring the temperature of the medium which convects the heat from the HTC, the rate of heat transfer from a fluid bed is adjusted by varying a parameter of the periodically varying velocity of the fluidizing gas in a HTC. The duration of high velocity gas flow sub-periods and low velocity gas flow sub-periods may also be altered according to a preset program to provide the desired heat transfer in said heat transfer chamber, i.e. for said temperature to reach the preset value.
When trying to establish stable conditions in processes utilizing fluidized bed reactors, a man familiar with this technology normally aims to avoid all types of variations in the system, not to cause them, Surprisingly it has now been found out that the heat transfer in the fluidized bed reactor can be controlled by expressly periodically varying the flow velocity using an appropriate flow function in a heat transfer chamber therein.
An observation behind this invention is that, due to the heat capacities involved in the system, such as metal tubes, fluid medium etc., it is possible to use periodically varying control procedures. Moreover, by using a periodically varying control procedurexe2x80x94with a not too long periodxe2x80x94an ideally controllable operation can be obtained.
This invention is cost-effective and can easily be applied to practice, because in many cases it can be made operational by only minor changes in the existing HTC gas velocity control equipment.
By the procedure disclosed herein the response time of the heat transfer system-is short, because the time constant of the gas flow is of the order of a few seconds and that of the heat transfer surfaces typically also at most only some tens of seconds. By using different values for the parameters of the periodical velocity of the fluidizing gas flow, a wide control range, e.g. from 100% to 25%, can be obtained.